Figure 1. (a) Axis specification of Drosophila melanogaster. Progressively older egg chambers are shown from left to right: germarium, stages 1–6, stages 7–9, and stage 10. In the germarium, an asymmetric division of a germline stem cell gives rise to a 16-cell cyst, consisting of 15 nurse cells and the pro-oocyte. This cyst becomes surrounded by somatically derived follicle cells forming the egg chamber or follicle. The oocyte nucleus is surrounded by Gurken protein (cyan). At stages 1–6 and stages 7–9 egg chambers, plus and minus indicate the orientation of microtubules. By stage 7, both anterior and posterior polar follicle cell fates (apfc and ppfc) have been determined and the oocyte nucleus migrates to a dorsal-anterior position where the dorsal signalling occurs. At stage 10 (mid-oogenesis) Gurken protein is distributed in an anterior-to-posterior gradient on the dorsal side. (Arrow indicates the site of cross section for schematic representation of dorsal-ventral polarity formation shown in (b)). (b) Cross-section of a stage-10 egg chamber. On the dorsal side, the tor/DER receptor is activated by Gurken causing the follicle cells to differentiate a dorsal fate. At the ventral side, the tor/DER signal is not activated. Wind and Ndl are expressed uniformly in the follicular epithelium, and Pipe expression is restricted ventrally (roughly one-third of the DV circumference). Ndl protein is distributed uniformly on the outside of the plasma membrane surrounding the oocyte. Wind and Pipe function defines a ventral positional cue that is transmitted to the developing embryo after fertilization.

Figure 2. Model of the establishment of the embryonic dorsal-ventral pattern. (a) Schematic representation of the Dorsal nuclear gradient in a blastoderm stage embryo. Dorsal side is up, ventral side is down. The boxed area is enlarged in the schematic in (b). (b) A schematic representation of the maternally-encoded, embryonic dorsal-ventral signal transduction pathway. Position cue represents the ventral signal determined in the follicle cells during oogenesis. See text for details. (vm = vitelline membrane, pm = plasma membrane.)

Figure 3. The fate map of the Drosophila embryo and the expression of early zygotic dorsal-ventral patterning genes. Subdivision of the dorsal-ventral axis is determined by the nuclear gradient of Dorsal protein. The activation of the zygotic genes depends on the threshold concentration of Dorsal protein and on the differential affinities of Dorsal binding sites in the promoters of the zygotic genes. The Dorsal protein inhibits the expression of tld, dpp, and zen in the ventral region.